Optical measurement systems exist in many variations. Common to these systems is that a beam of light is directed to the sample and light is captured from the sample. The captured light may be light reflected from the sample, transmitted through the sample and/or light emitted from the sample in response to the incoming beam such as fluorescence.
Octave bandwidth supercontinuum (SC) has been successfully generated directly through non-linear fibers, such as microstructured fibers, tapered standard fibers, and tapered microstructured fibers by pumping the fiber with pulsed lasers (often in a MOPA configuration) as input. Such a spectrally broad continuum source is potentially useful in many measurement systems, such as optical coherence tomography (OCT), optical frequency metrology, fluorescent microscopy, coherent anti-Stokes Raman scattering (CARS) microscopy, and two-photon fluorescence microscopy. Unfortunately, for those experiments, the large amplitude fluctuations of conventional continuum sources limit accuracy and/or sensitivity. Previous studies of SC generation have shown that the SC generation process is very sensitive to quantum noise, technical noise, and specific parameters such as the input wavelength, time duration, and chirp of the input laser pulses. A light source derived from a stable continuum would generally improve the usefulness of SC sources.
Continuum generation in conventional holey, photonic crystal, or tapered single-mode long fibers is complex and can contain significant sub-structures in the time and frequency domains leading to undesirable and unevenly distributed noise and instability for different wavelength regions. Usually, the amplitude of the continuum shows large fluctuations with significant excess white-noise background, which can be revealed with a fast detector and RF spectrum analyzer (RFSA) measurement.
A common approach to wavelength conversion is to generate a supercontinuum, then spectrally slice off part of the continuum and use this slice as the light source for the microscopy setup. However, the selected continuum likely contains large amplitude fluctuations (noise), which may not be suitable for some applications.
In U.S. Pat. No. 7,403,688, noise from the SC source is reduced by tapering the non-linear fiber and using a femto-second pulse source which gives rise to so-called soliton fission. The abstract of this patent states: “The longitudinal variation of the phase-matching conditions for Cherenkov radiation (CR) and four-wave mixing (FWM) introduced by DMM allow the generation of low-noise supercontinuum.” Tapering requires either a post processing technique or variation of diameter of the fiber during production which may complicate the production of the SC light source, and the small cross section of a taper may limit the amount of light which can be safely transmitted. Furthermore, femto-second pump sources are often relatively complex and expensive.
In US2011/0116282 a light source apparatus having a base structure capable of generating SC light and further having a structure that enables the shaping of the spectral waveform of the SC light, power adjustment of the SC light, or adjustment of the frequency of repetition of the pulse train that contains the SC light is described. The light source apparatus of US2011/0116282 comprises a SC fiber pumped at wavelengths at about 1550 nm and the frequency of repetition of a SC optical pulse train from the light source lies between 1 MHz or more, but at 100 MHz or less. Throughout US2011/0116282, noise is only discussed in relation to single pulses, and it is described that the noise characteristic of the pulse light P1 is not influenced. In relation to the noise characteristic of the SC optical pulse train P2, it is mentioned that low noise detection is possible through synchronization with an optical detector disposed outside the light source apparatus. Noise spectra from SC light sources using different pump wavelengths differ, and thus noise suppression may differ. US2011/0116282 refers to femtosecond pulse trains P1. Such pump sources are often relatively complex and expensive.